Carburizing austempering process

ABSTRACT

A novel combination of heat treatment steps includes the steps of carburizing a component fabricated of a medium carbon alloy steel at an elevated temperature for between three and six hours, subjecting the component to an austempering bath and holding it there for between fifteen and two hundred forty minutes and finally cooling the component to room temperature to allow martensitic transformation. These steps may be followed with cryogenic treatment to reduce retained austenite if needed. The process produces components with low distortion, high surface hardness, from HRC 56 to 62, and high surface compressive residual stress.

CROSS REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/314,521, filed Mar. 29, 2016, which is hereby incorporated in its entirety herein by reference.

FIELD

The present disclosure relates to a novel heat treatment process for medium carbon alloy steels and more particularly to a novel heat treatment process for medium carbon alloy steels comprehending carburizing, austempering and cooling steps.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.

Post fabrication treatment of fabricated metal parts such as gears, shafts, sprockets, bearings and similar components is commonplace. The usual reason for such treatment is a desire or need to increase the strength and durability of the part and most processes involve heating the part, often in a controlled atmosphere, followed by controlled cooling. There are many suitable steel alloys and numerous heat treating processes which may be combined to achieve improved hardness and durability of metal parts.

Unfortunately, there are as well many complications that arise both from alloy choice and heat treatment selection. For example, certain heat treatment processes are limited to certain alloys, that is, medium carbon steels may only be heat treated by certain processes and these limited processes may not achieve a desired final condition such as hardness. As another example, the distortion of a component during heat treating generally corresponds to the carbon content of the alloy. This consideration encourages the use of lower carbon steels which, however, may not be capable of achieving a final desired hardness. A further consideration relates to quenching speed. Rapid quenching, while desirable from hardness and microstructure standpoints, may result in both distortion of the component and the production of residual internal stress. Finally, if a portion of the heat treatment process achieves an austenitic to martensitic transformation, it may result in a change in size of the component.

Obviously, the foregoing consequences of heat treatment are undesirable and present an engineering challenge of both maximizing the desirable metal and process characteristics while minimizing those undesirable characteristics for a certain component and application.

SUMMARY

The present invention comprehends a novel combination of heat treatment steps including the steps of carburizing a component fabricated of a medium carbon alloy steel at an elevated temperature for between three and six hours, subjecting the component to an austempering bath and holding it there for between fifteen and two hundred forty minutes (four hours) and finally cooling the component to room temperature to allow martensitic transformation. These steps may be followed with a cryogenic treatment to reduce retained austenite if needed. The process produces components with low distortion, high surface hardness, from HRC 56 to 62, and high surface compressive residual stress.

Thus it is an aspect of the present invention to provide a method of heat treating metal components.

It is a further aspect of the present to provide a method of heat treating metal components fabricated of a medium carbon alloy steel.

It is a still further aspect of the present invention to provide a method of heat treating metal components including the step of carburizing the component.

It is a still further aspect of the present invention to provide a method of heat treating metal components fabricated of a medium carbon alloy steel including the step of carburizing the component.

It is a still further aspect of the present invention to provide a method of heat treating metal components including the step of subjecting the component to an austempering bath.

It is a still further aspect of the present invention to provide a method of heat treating metal components fabricated of a medium carbon alloy steel including the step of subjecting the component to an austempering bath.

It is a still further aspect of the present invention to provide a method of heat treating metal components including the step of cooling the component to room temperature to allow martensitic transformation.

It is a still further aspect of the present invention to provide a method of heat treating metal components fabricated of a medium carbon alloy steel including the step of cooling the component to room temperature to allow martensitic transformation.

It is a still further aspect of the present invention to provide a method of heat treating metal components including the optional step of cryogenically treating the component.

It is a still further aspect of the present invention to provide a method of heat treating metal components fabricated of a medium carbon alloy steel including the optional step of cryogenically treating the component.

Further aspects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIG. 1 illustrates a plurality of exemplary mechanical components utilized in motor vehicle powertrain devices that may be treated by the carburizing and austempering process described herein; and

FIG. 2 is a flowchart setting forth in sequence the steps of the carburizing and austempering process according to the present invention.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

Referring first to FIG. 1, several exemplary mechanical components typically utilized in motor vehicle powertrain devices that benefit from the carburizing and austempering process described herein are illustrated. For example, there is illustrated a sliding camshaft 10, a spur gear 12, a helical gear 14, a helical face gear 16, planetary gear carrier 18, a splined shaft 20, a chain sprocket 22 and a blocker ring 24 of a transmission synchronizer assembly which all may undergo the process described below and which will thus exhibit high surface hardness, low post fabrication distortion and surface compressive residual stress. It will be appreciated that this delineation of treated components is not, and is not intended to be, inclusive but merely exemplary as various and numerous other metal components can and will benefit from the carburizing and austempering process described herein.

With reference now to FIG. 2, a flowchart illustrating the plural steps of the present inventive method is generally designated by the reference number 30. At the outset, it should be appreciated that a particular steel alloy composition has been found to provide desirable final strength and hardness characteristics in components subjected to the carburizing and austempering process described herein. Such alloy composition has a carbon content of between about 0.20% to 0.60%, chromium content of between about 0.50% to 2.0%, manganese content of between 0.0% to about 3.5%, nickel content of between 0.0% to about 0.8%, vanadium and/or niobium content of between 0.0% to about 0.2%, molybdenum content of between 0.0% to about 0.1%, silicon, a maximum of about 0.1%, sulphur, a maximum of about 0.1% and phosphorus, a maximum of about 0.05%. A commercially available, standard steel alloy essentially similar to this composition is SAE 4340.

After the fabrication and final finishing, such as, for example, machining or grinding, of a component in a step 32, the component is subjected to a carburizing or carbonitriding process in an oven at a step 34. Preferably, the component is exposed to a carburizing temperature of between about 850° C. and 1200° C. for between about three and six hours. The purpose of this carburizing step 34 is to produce an exterior shell or layer of the component that is rich in carbon. The carbon potential near the surface of the component is preferably between about 0.7% and 1.2%. The carburizing step 34 also depresses the martensitic start temperature so that the following austempering step 36 can be carried out at the traditional temperature range. The carburizing step 34 may be accomplished by either a gas carburizing process, a low pressure carburizing process or a carbonitriding process.

Next, in an austempering step 36, the component is cooled in an oil or salt bath at a temperature of between about 250° C. and 450° C. for between about fifteen minutes and two hundred forty minutes (four hours). As noted above, because the increased carbon content on and near the surface of the component achieved in the step 32 depresses the martensitic start temperature, the austempering step 34 produces bainite and retained austenite on and near the surface of the component. The austempering step 36 thus has two purposes and objectives: the first is to reduce distortion of the component and the second is to provide a final microstructure of bainite, martensite and retained austenitie on the surface of the component, with the martensitic interior as described above.

In a final necessary cooling step 38, the components are cooled to room (ambient) temperature by exposure to atmospheric air or other gasses or liquids. The retained austenite is partially transformed to martensite when cooling from the austempering temperature to room temperature in the cooling step 38.

In a final, optional step, that may or may not be necessary depending upon the final desired hardness and other aspects of the component such as dimensional requirements, a cryogenic treatment step 40 may be undertaken. In this step 40, the component is subjected to a cryogenic bath of, for example, liquid nitrogen at −185° C., for a time sufficient to fully cool the component. The cryogenic treatment step 40 enhances surface hardness and transforms any untransformed, retained austenite into martensite. The process 30 concludes at an end or stop step 42.

The forgoing process steps when utilized on a metal component having an alloy composition substantially as stated above provide a surface hardness of HRC 58 minimum and a core hardness of HRC 45 minimum at the thickest wall locations. Typically, the surface hardness may be HRC 62. In addition to such surface and core hardness, a component which has undergone the foregoing process exhibits low post fabrication distortion and high surface compressive residual stress.

The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention. 

What is claimed is:
 1. A heat treatment process for a metal component, comprising the steps of: carburizing the metal component at an elevated temperature for between three and six hours, subjecting the component to an austempering bath for between fifteen minutes and four hours, and cooling the component to room temperature thereby allowing martensitic transformation.
 2. The heat treatment process of claim 1, wherein the carburizing step is achieved by one of gas carburizing, low pressure carburizing process and carbonitriding.
 3. The heat treatment process of claim 1, wherein the austempering step is achieved in an oil or salt bath at a temperature of between 250° C. and 450° C.
 4. The heat treatment process of claim 1 wherein the cooling step is achieved by exposing the component to atmospheric air.
 5. The heat treatment process of claim 1, further including the step of subjecting the component to a cryogenic bath to cool the component to a temperature of the cryogenic bath.
 6. The heat treatment process of claim 5 wherein the cryogenic bath is liquid nitrogen.
 7. The heat treatment process of claim 1 wherein the metal component is fabricated of SAE 4340 steel.
 8. A heat treatment process for a medium carbon alloy steel component, comprising the steps of: carburizing the medium carbon alloy steel component at an elevated temperature for between three and six hours, subjecting the component to an austempering bath for between fifteen minutes and four hours, cooling the component to room temperature thereby allowing martensitic transformation, and subjecting the component to a cryogenic bath to cool the component to a temperature of the cryogenic bath.
 9. The heat treatment process of claim 8, wherein the carburizing step is achieved by one of gas carburizing, low pressure carburizing process and carbonitriding.
 10. The heat treatment process of claim 8, wherein the austempering step is achieved in one of an oil bath and salt bath at a temperature of between 250° C. and 450° C.
 11. The heat treatment process of claim 8 wherein the cooling step is achieved by exposing the component to atmospheric air.
 12. The heat treatment process of claim 8 wherein the medium carbon alloy steel component is fabricated of SAE 4340 steel.
 13. A heat treatment process for a medium carbon alloy steel component, comprising the steps of: carburizing a medium carbon alloy steel component at an elevated temperature for between three and six hours to produce a layer near a surface of the component having between 0.7% and 1.2% carbon, subjecting the component to an austempering bath for between fifteen minutes and four hours to produce bainite and retained austenite on and near the surface of the component, and cooling the component to room temperature thereby allowing martensitic transformation.
 14. The heat treatment process of claim 13, wherein the carburizing step is achieved by one of gas carburizing, low pressure carburizing process and carbonitriding at a temperature of between 850° C. and 1200° C.
 15. The heat treatment process of claim 13, wherein the austempering step is achieved in an oil or salt bath at a temperature of between about 250° C. and 450° C.
 16. The heat treatment process of claim 13 wherein the cooling step is achieved by exposing the component to atmospheric air.
 17. The heat treatment process of claim 13 wherein the medium carbon alloy steel component is fabricated of SAE 4340 steel.
 18. The heat treatment process of claim 13, further including the step of subjecting the component to a cryogenic bath to cool the component to the temperature of the cryogenic bath. 